Conventionally, when images are formed with copying machines, laser beam printers, etc., the Carlson Method has been generally used (U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809, "Electrophotography," p22-p41, R. M. Shaffert, 1965, The Focal Press).
FIG. 2 shows a schematic view of an apparatus for a conventional method of forming fixed images. In the conventional method, after the electrostatic latent image formed on a photoconductor by optical means is developed in a developing process, it is transferred to a recording medium such as recording gaper in a transfer process and then fixed into the final image generally with heat and pressure in a fixing process. As the photoconductor is repeatedly used, a cleaning device is provided for cleaning the residual toner after the transfer process with its rotation.
In the conventional method of forming fixed images, however, through the processes from the formation of the electrostatic latent image up to its fixing onto the recording medium, the temperature of the heating element of the fixing device has to remain at a very high level (usually around 200.degree. C.) and further a relatively high pressure is required (usually between 2.0 and 6.0 kg/cm). On the other hand, since both the photoconductor and the developing device have to be maintained at around room temperature, a considerable distance has to be maintained between the fixing device and the developing device, which necessitates making the machine larger. In addition, it is necessary to force the removal of the generated heat from the system, but the noise produced by the forced radiation device is not negligible.
Further, in the conventional method of forming fixed images, since the fixing section works independently and at such a high temperature of around 200.degree. C., as mentioned above, expensive heat-resistant materials such as heat-resistant resins, heat-resistant rubbers, etc. have to be provided around the fixing device.
When the fixing is carried out at a high temperature, it is subject to problems such as curling and jamming of the paper, etc. In addition, it is pointed out that a fixing failure may take place due to the heat absorbed by the paper, depending upon its thickness. Further, if the fixing requires a high temperature, it takes more time to reach the set temperature so that a quick printing becomes impossible. In such a case, therefore, this method is unsuitable for devices such as a facsimile which requires quick printings.
As for solving these problems, a device for carrying out low temperature fixing using a cold pressing method (Japanese Patent Laid-Open No. 159174/1984) is known. In this reference, however, although the fixing temperature is low, the nip pressure has to be elevated normally to not less than 4 kg/cm in this method, making the machine heavier. Moreover, it poses problems in the gloss of the images, deformation of the paper copy sheets and an insufficient fixing strength. As for a fixing device for fixing images at such a low nip pressure of less than 4 kg/cm, a heat roller method is known, for example, but it has been pointed out that the fixing temperature needs to be maintained at not less than 120.degree. C.
Under the circumstances, the development of a fixing device that can fix images at a low temperature and at a low nip pressure is highly desired, but it has not yet been developed. Further, as regards toners to be indispensably used for the image formation, since they have been confined to those made from a thermoplastic resin dispersed with additives such as coloring agents, charge control agents, releasing agents, etc., and pulverized, there have been limitations on the molecular weight, the softening point of the thermoplastic resin for use in the toner from the aspect of storage stability, thereby posing limitations on the further pursuit of low temperature fixing.
From these standpoints, the development of a novel method of forming fixed images as well as a matching toner thereto is in demand.